1) You can find the number of unique anonymous visitors in Marketo in the Web Page Activity report. Make sure you change the activity source to Anonymous Visitors (including ISPs)

2) Then, in the People Performance report you can get the number of newly created leads in the same period. Make sure you include in the smart list tab of the report "Original Source Type is not salesforce.com;List Import" (if you're using salesforce) to exclude leads created from CRM and imported into Marketo.

You can work out your ratio based on the aggregated figures from both those reports

Catina, just a note on Jay's recommendation (which is indeed the best you can do with the available data): remember that a single human lead can have 0 or more anonymous web sessions and 0 or more associated web sessions in action at the same time.

For example, if I follow a PPC link on my desktop, then re-Google (or navigate directly) on my mobile and fill out a form, the first session is anonymous (and would appear in a WPA report set to source = anonymous) while the second session is associated (and would appear in the other WPA report set to source = known).

Therefore, on the same day, the same human is represented by both a previously-anonymous conversion and a still-anonymous session. The numbers will converge toward real truth over time (if you go back and re-run the report) but must be considered "directional" rather than exact.